What are these grinders? I see people talking about them but I search grinders in NEI I don't see anything.
The ghasts won't spawn inside other mobs, so if you have, for example, a 6x6 area with a spider in the middle, there won't be enough room for a ghast. Assuming your other shards are tier 5, they'll be spawning fast enough to stop the ghasts.Soul shards care not. All the other spawners work fine - I've currently got spiders, angry zombies, and endermen going. Soul shards will happily spawn in mid-air.
Again, Soul Shards. a Tier 5 soul shard will very happily spawn things in mid air.Something to note, ghasts spawn on walls, they don't spawn in the air like anything else
I have a feeling that this is your problem
To be honest ghasts are easier to kill with quicksand. I built a spawner for them once to test things and show you could kill anything with quicksand if you use it right.
The problem is that dont like spawning in small spaces and they dont like to come down making it hard for the conveyors to grab one.
I'll see if I can find the thread about ghast murdering.
Actually looking at my old build and your OP image.
I think you can fix that just be removing that shaft of blocks over the spawner.